PPW4 instead of PPW2 / subsequent change of thermal protection proof

For our new building (KfW40), the exterior walls are constructed with aerated concrete blocks, then insulated and subsequently clad with bricks. In the energy calculation available to us and according to the building specification, blocks of strength class 2 (Lambda value of 0.10) are to be used. However, during a site visit, we found that PPW 4 (Lambda value of 0.16) and partly also PPW 6 are being used. Upon inquiry, the site manager informed us that the structural analysis required the change and that the thermal protection verification would still be revised.

Is this plausible and acceptable?

Unlike the energy calculation, the structural calculation is not available to us. The structural and energy calculations come from the same structural engineer with whom the construction company always cooperates. I have a strong suspicion that an incorrect building material was ordered and used, and now the thermal protection verification is being retroactively adjusted. Or am I being too critical?

What can or must we do?

That's not entirely far-fetched.
Contractually agreed is Kfw40 or the stone?

The contract defines both KfW40 and the building material. Changes to "equivalent" material would need to be agreed upon.

Wrong building material ordered probably rather unlikely. More likely ordered consciously because it’s cheaper. Or truly necessary for structural reasons. Definitely get the structural engineering report. You won't be able to do much with it. But the person who might renovate your house in 30 years and wants to remove a wall will be grateful!

As long as you still achieve the Kfw 40 standard, that’s okay.
There will certainly be a clause in the contract like: "if technically necessary, deviations may be made"
But they should have at least informed you, a mild reprimand is justified.

Best regards,

Andreas